1. Field of the Invention
The present invention relates to a voltage controlled oscillator, and more particularly, to technology for designing a multi-phase multi-band voltage controlled oscillator using transformer coupling which may be used in an IC or the like using CMOS technology.
2. Description of the Related Art
When a voltage controlled oscillator is designed according to the related art, inductors are used as the inductance components of an LC tank.
When a quadrature-phase oscillator having four phases is designed so as to increase the number of oscillation phases, two differential oscillators generating two phase signals may be connected so as to have four phases.
In this case, an occupied area and power consumption may increase, and the performance of the oscillator may be degraded by substrate coupling.
To reduce the impact of substrate coupling, a method using a back gate has been proposed. However, the method has a disadvantage in that the magnitude of generated signals is not sufficiently large. Furthermore, when two oscillators are used as described above, the occupied area and power consumption may increase two-fold. Such a problem is not easy to solve.
In the case of an existing voltage controlled oscillator using transformers, great effort has been made to improve the performance of an LC tank and reduce power consumption through coupling. However, since the number of transformers used therein is not accordingly reduced, there is a limit to reducing the occupied area.
FIGS. 1A and 1B are block diagrams illustrating a state in which two differential oscillators are cross-coupled to thereby obtain a quadrature-phase voltage controlled oscillator according to the related art.
Referring to FIGS. 1A and 1B, two differential oscillators are cross-coupled in such a manner that four oscillation nodes have a 90-degree phase difference from each other.
Referring to FIG. 1A, an oscillator core 11 includes an LC tank and coupling transistors M1c and M2c. The LC tank includes NMOS transistors M1 and M2, inductors L, and varactor diodes C. The coupling transistors M1c and M2c are required for connecting two oscillator cores 11 to each other. Through the above-described configuration, the oscillator core 11 has four ports Vi+, Vi−, Vo+, and Vo−. The two oscillator cores 11 having four ports are cross-coupled in such a manner that the respective oscillation nodes have a 90-degree phase difference from each other. An oscillation signal may be varied by a control voltage Vcon, and bias current may be controlled by a bias voltage Vbias.
The quadrature-phase voltage controlled oscillator of FIG. 1A has disadvantages in that the coupling transistors may increase the current and a substrate noise effect. To overcome such disadvantages, the quadrature-phase voltage controlled oscillator of FIG. 1B adopts a circuit structure using a back gate instead of the coupling transistors.
Referring to FIG. 1B, an oscillator core 22 includes an LC tank, back-gate resistors Rb and capacitors Cb. The LC tank includes NMOS transistors M1 and M2, inductors L, and varactor diodes C. The back-gate resistors Rb and capacitors Cb are required for connecting two oscillator cores 22 to each other. Through the above-described configuration, the oscillator core 22 has four ports Vi+, Vi−, Vo+, and Vo−. The two oscillator cores 22 having four ports are cross-coupled in such a manner that the respective oscillation nodes have a 90-degree phase difference from each other. An oscillation signal may be varied by a control voltage Vcon, and bias current may be controlled by a bias voltage Vbias.
The above-described structure is frequently used for implementing a quadrature-phase voltage controlled oscillator. However, since two differential oscillators using the LC tank including inductors or transformers, varactor diodes, and varactor banks are required, the occupied area and the power consumption inevitably increase.
In a wireless communications field, there have been many advances in digital RF transceivers as well as in existing analog transceivers. Furthermore, software defined radio (SDR) transceivers have been recently developed. The current trend is for such transceivers to support multiple bands and multiple modes.
In order to implement such a structure, a reconfigurable circuit which operates smoothly should be provided. When such a structure is implemented, there are technical difficulties in designing a voltage controlled oscillator in a frequency synthesizer such that the voltage controlled oscillator can cover all frequency domains. Several voltage controlled oscillators may be used for each frequency. In this case, however, the occupied area and the power consumption inevitably increase.
Most transceivers which are currently used require in-phase and quadrature phases (I/Q). In order to generate the in-phase and quadrature phases, a method using a divider is frequently used. However, the method has a disadvantage in that there are difficulties in designing the divider operating at a high frequency as the frequency increases. In this case, an oscillator designed to have quadrature phases may be applied, without using the divider.
Therefore, a multi-phase multi-band voltage controlled oscillator which has a small area and operates at low power may be usefully applied to an RF transceiver supporting multiple bands and multiple modes.
A low-power quadrature oscillator may be designed by using transformer coupling. However, since there is a disadvantage in that the occupied area is not reduced, the quadrature oscillator needs to be configured so as to have four ports oscillating in one transformer. Such a method makes it possible to design a quadrature oscillator having a small area.